Python GUI guizero: Box layout header/sidebar

from guizero import App, Box, PushButton, Text, TextBox, ButtonGroup, Slider
app = App( height=600, width=800)
"""
This GUI is split into 3 Box(es).
- Header, Sidebar, and Content
"""
#--------------
boxHeader = Box(app, align="top", width="fill", height=150, border=1)
boxHeader.bg="yellow"
boxHeader.text_color="blue"
# - add widgets to Box
txtHeader_h2 = Text(boxHeader, text="Header", width="fill")
txtHeader_h2.text_size = 24
txtHeader_h2.bg="#000000"
txtHeader_h2.text_color="white"

"""
button01_2 = PushButton(boxHeader, text="Red")
button01_2.bg="red"
button01_2.text_color="white"
"""
txtHeader_slider = Text(boxHeader, text="Change contrast")

slider = Slider(boxHeader, width=400, height=20)
slider.value = 50
slider.bg = "#ff4a6e"
slider.text_color = "#000000"
slider.text_size = 14
#---------------
boxSidebar = Box(app, align="left", width=160, height="fill", border=1)
boxSidebar.bg="#DDDDDD"
boxSidebar.text_color="#000000"

# - add widgets to Box
txtSidebar_h2 = Text(boxSidebar, text="Sidebar", width="fill")
txtSidebar_h2.text_size = 18
txtSidebar_h2.bg="cyan"
txtSidebar_h2.text_color="white"

txtSidebar_sub1 = Text(boxSidebar, text="News type ..")
txtSidebar_sub1.text_size = 14

choice02_1 = ButtonGroup(boxSidebar, options=["World", "National", "Local"], selected="World")
#---------------
boxContent = Box(app, align="right", width="fill", height="fill", border=1)
boxContent.bg="white"
boxContent.text_color="#000000"
# - add widgets to Box
txtContent_h2 = Text(boxContent, text="Main Content", width="fill")
txtContent_h2.text_size = 20
txtContent_h2.bg="#000000"
txtContent_h2.text_color="white"

txtContent_h3 = Text(boxContent, text="Please enter your news ..", width="fill")
txtContent_h3.text_size = 14
txtContent_h3.bg="white"
txtContent_h3.text_color="blue"

txtContent_in = TextBox(boxContent, text="", multiline=True, height=8, width=90)
txtContent_in.text_size = 14
txtContent_in.bg="#CCCC00"
txtContent_in.text_color="white"

buttonSubmit = PushButton(boxContent, text="Submit story ..")
buttonSubmit.bg="red"
buttonSubmit.text_color="white"
#   -------
txtContent_in.focus() # put text cursor in TextBox, ready for user input ..
app.display()